Why not look for a 73 Dart setup? They made a bizillion 73 Darts! The smaller bumper used the traditional fixed steel brackets. Those can be slotted to tuck the bumper in even closer.
The trick is to find a way to attach it to the frame rails.
In my 74 Duster, I made 2 steel plates from 14 gauge metal, then I welded course threaded nuts to them. I then inserted them into the frame rail, then welded them in place. This allowed me to switch to an earlier bumper for the cleaner look.
If you are married to the idea of working with what you have, YES, the shock absorber setup can be modified. The tubes may have springs inside, I cannot recall the innards. Remove the spring, slide them closer together and weld them in place to keep them secure.